Oxygen monitoring apparatus and methods of using the apparatus

ABSTRACT

Apparatus or systems which employ luminescence quenching to produce an oxygen concentration indicative signal. Components of such systems include: (1) an airway adapter, sampling cell, or the like having a casing and a sensor which is excited into luminescence with the luminescence decaying in a manner reflecting the concentration of oxygen in gases flowing through the airway adapter or other flow device and is in intimate contact with a window in the casing; (2) a transducer with has a light source for exciting a luminescable composition in the sensor into luminescence, a light sensitive detector for converting energy emitted from the luminescing composition as that composition is quenched into an electrical signal indicative of oxygen concentration in the gases being monitored, and a casing which locates the light source and detector in close physical proximity to the window but on the side thereof opposite the sensor; and (3) subsystems for maintaining the sensor temperature constant and the temperature of the window above condensation temperature and for processing the signal generated by the light sensitive detector. Airway adapters, sampling cells, and transducers for such systems.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to the monitoring of oxygenconcentration and, more particularly, to novel, improved methods andapparatus for monitoring the concentration of oxygen in respiratory andother gases.

BACKGROUND OF THE INVENTION

[0002] The most common cause of anesthetic and ventilator relatedmortality and morbidity is inadequate delivery of oxygen to a patient'stissues. Therefore, the monitoring of static inspired oxygenconcentration has long been a safety standard of practice to ensuredetection of hypoxic gas delivery to patients undergoing surgery and tothose on mechanical ventilators and receiving supplemental oxygentherapy. However, monitoring the static inspired fraction of inhaledoxygen does not always guarantee adequate oxygen delivery to the tissuesbecause it is the alveolar oxygen concentration that eventually enrichesthe blood delivered to the cells.

[0003] It is this alveolar gas phase that is interfaced with pulmonaryperfusion which, in turn, is principally responsible for controllingarterial blood gas levels. It is very important that the clinician knowthe blood gas levels (partial pressure) of oxygen (pO₂) and carbondioxide (pCO₂) as well as the blood pH. Blood gas levels are used as anindication of incipient respiratory failure and in optimizing thesettings on ventilators. In addition, blood gas levels can detectlife-threatening changes in an anesthetized patient undergoing surgery.

[0004] The traditional method for obtaining arterial blood gas values ishighly invasive. A sample of arterial blood is carefully extracted andthe partial pressure of the gases is measured, using a blood gasanalyzer. Unfortunately, arterial puncture has significant inherentlimitations: (1) arterial puncture requires a skilled health careprovider and it carries a significant degree of patient discomfort andrisk, (2) handling the blood is a potential health hazard to the healthcare provider, (3) significant delays are often encountered beforeresults are obtained, and (4) measurements can only be madeintermittently.

[0005] Non-invasive methods for estimating blood gas levels areavailable. Such methods include the use of capnography (CO₂ analysis).These methods employ fast gas analyzers at the patient's airway and givea graphic portrayal of breath-by-breath gas concentrations and,therefore, can measure the peak exhaled (end tidal) concentrations ofthe respective respired gases. Although gradients can occur between theactual arterial blood gas levels and the end tidal values, this type ofmonitoring is often used as a first order approximation of the arterialblood gas values.

[0006] Other techniques have been utilized for assessing patient bloodgas levels with mixed results. Transcutaneous sensors measure tissue pO₂and pCO₂ diffused through the heated skin surface. This type of sensorhas a number of practical limitations including a slow speed of responseand difficulty of use.

[0007] Pulse oximetry is widely used to measure the percentage ofhemoglobin that is saturated with oxygen. Unfortunately, it does notmeasure the amount of dissolved oxygen present nor the amount of oxygencarried by the blood when the hemoglobin levels are reduced. This isimportant because low hemoglobin levels are found when there is asignificant blood loss or when there is insufficient red blood cellinformation. In addition, pulse oximeter readings are specific to thepoint of contact, which is typically the finger or ear lobe, and may notreflect the oxygen level of vital organs during conditions such as shockor hypothermia.

[0008] Oxygraphy measures the approximate concentration of oxygen in thevital organs on a breath-by-breath basis and can quickly detect imminenthypoxemia due to decreasing alveolar oxygen concentration. For example,during hypoventilation, end tidal oxygen concentration changes morerapidly than does end tidal carbon dioxide. During the same conditions,pulse oximetry takes considerably longer to respond. Fast oxygenanalysis (oxygraphy) can also readily detect inadvertent administrationof hypoxic gas mixtures.

[0009] Oxygraphy reflects the balance of alveolar O₂ available duringinspiration minus the O₂ uptake secondary to pulmonary perfusion. Anincreasing difference between inspiratory and end tidal oxygen values isa rapid indicator of a supply/demand imbalance which could be a resultof changes in ventilation, diffusion, perfusion and/or metabolism of thepatient. This imbalance must be quickly corrected because failure tomeet oxygen demand is the most common cause of organ failure, cardiacarrest, and brain damage. Oxygraphy provides the earliest warning of thedevelopment of an impending hypoxic episode.

[0010] Oxygraphy has also been shown to be effective in diagnosinghypovolemic or septic shock, air embolism, hyperthermia, excessive PEEP,CPPR efficacy, and even cardiac arrest. During anesthesia, oxygraphy isuseful in providing a routine monitor of preoxygenation(denitrogenation). It especially contributes to patient safety bydetecting human errors, equipment failures, disconnections,misconnections, anesthesia overdoses, and esophageal intubations.

[0011] Combining the breath-by-breath analysis of oxygen with themeasurement of airway flow/volume as outlined in U.S. Pat. Nos.5,347,843 and 5,379,650 gives another dimension to the clinical utilityof oxygraphy. This combination parameter, known as oxygen consumption(VO₂), provides an excellent overall patient status indicator. Adequatecardiac output, oxygen delivery, and metabolic activity are allconfirmed by oxygen consumption because all of these physiologicalprocesses are required for oxygen consumption to take place. Oxygenconsumption is also useful in predicting ventilator weaning success.

[0012] A metabolic measurement (calorimetry) includes determination of apatient's energy requirements (in calories per day) and respiratoryquotient (RQ). Interest in the measurement of caloric requirements hasclosely paralleled the development of nutritional support. For example,the ability to intravenously provide all the necessary nutrition tocritically ill patients has only been accomplished within the last 25years. Along with the realization that we need to feed patients, hascome the need to know how much to feed them, what kind of nutrients(carbohydrates, lipids, protein) to feed them, and in what ratio thenutrients need to be supplied. The only true way to measure the caloricrequirements of patients and to provide a non-invasive qualityassessment of their response to nutrition is with indirect calorimetry.Airway O₂ consumption and CO₂ production can be measured non-invasivelyand provide a basis for the computations needed for a measurement ofindirect calorimetry, a direct measurement of the metabolic status ofthe patient, and the patients' respiratory quotient.

[0013] With the above clinical need in mind, it is important to ensurethat clinicians have the proper equipment to monitor breath-by-breathoxygen. While there are adequate devices for measuring static levels ofoxygen, the measurement of breath-by-breath (fast) airway oxygenconcentration requires more sophisticated instruments. Very few of thesedevices can be directly attached to the patient airway. Instead, mostrequire the use of sampling lines to acquire the gas and send it to aremote site for analysis. Fast airway oxygen monitors are typicallylarge, heavy, fragile instruments that consume considerable power. Theymust sample airway gases via a small bore plastic tube (sidestream) andremotely detect the oxygen gas as it passes from the airway to thesensor. The problems associated with this type of gas sampling are wellknown. Gas physics dictates painstaking, careful measurements becausewater vapor concentration pressure and temperature can vary within thepatient's airway and gas sample line. The presence of water and mucouscreates problems for long term patency of the sample tube. Also, thesample line acts like a low pass filter and affects the fidelity of themeasurement. Finally, the pressure variable delay introduced by thesample line creates difficulty in accurately synchronizing the airwayflow and oxygen concentration signals required to calculate oxygenconsumption.

[0014] On-airway (mainstream) monitoring of oxygen has the potential tosolve all of the above problems, especially when breath-by-breathmonitoring oxygen consumption measurements are made. However, most ofthe available fast oxygen sensors are simply too big, too heavy, toofragile, and/or otherwise not suited to be placed in line with apatient's breathing tube.

[0015] There are various other technologies which have been employed inmonitoring airway oxygen concentration. Some of the most widely used areelectrochemical sensors. These fall into two basic categories:polarographic cells and galvanic cells. These cells produce an electriccurrent proportional to the number of oxygen molecules which diffuseacross a membrane. The advantages of these types of sensors aresimplicity and low cost. The disadvantages of these types of sensorsinclude limited lifetime (chemistry depletes) and slow response (notbreath-by-breath). In some cases, these cells have demonstratedsensitivity to certain anesthetic agents, which introduces inaccuraciesinto the oxygen concentration measurement. Generally, this type ofsensor is too large to attach to the patient airway.

[0016] There have been a few reported developments where electrochemicalcell membranes were improved to enable faster response. There are alsosilicon micromachined cells using the principle of “Back Cell”electrochemical technology. Their time response approaches 150 ms butthey appear to be subject to the typical problems of this type of cell(i.e., stability and calibration).

[0017] Another popular medical oxygen sensor is the paramagnetic type.This sensor uses the strong magnetic property of oxygen as a sensingmechanism. There are two basic types of paramagnetic cells: static anddynamic. The static type is a dumbbell assembly suspended between thepoles of a permanent magnet. The magnetic forces of the surroundingoxygen molecules cause a torsional rotation of the dumbbell which can besensed optically and employed as a measure of oxygen concentration. Thedynamic type (see U.S. Pat. No. 4,633,705) uses a magneto-acousticapproach. This requires a gas sample and a reference gas that are mixedwithin an electromagnetic field. When the field is switched on and off,a pressure signal proportional to the oxygen content is generated. Thesignal can be detected by a differential microphone. The advantages ofthe paramagnetic sensor are good linearity and stability. The dynamictype has an inherently faster response than the static type. Both typesare subject to mechanical vibration, and the dynamic type has thedisadvantage of requiring a reference gas. Neither type is suitable foron-airway applications.

[0018] Zirconium oxide cells are frequently used in the automotiveindustry to measure oxygen concentration. The cell is constructed from asolid electrolyte tube covered by platinum electrodes. When heated toapproximately 800 degrees C., a voltage proportional to the logarithm ofthe ratio between a sample gas and a reference gas is generated. Theadvantage of this sensor are wide dynamic range, very fast response, andsimplicity. The high cell temperature is clearly a disadvantage as ispower consumption. Also, the cell is degraded in the presence ofanesthetic agents. Clearly, this type of cell cannot be used on apatient airway.

[0019] Ultraviolet absorption uses the principle that oxygen exhibitsabsorption properties in the ultraviolet part of the electromagneticspectrum (about 147 nm). This technique has been used in several medicalapplications but has never been reduced to commercial viability. Thereare numerous technical difficulties which make this a difficulttechnique for on-airway applications.

[0020] Mass spectrometers spread ionized gas molecules into a detectablespectrum according to their mass-to-charge ratios and can accordingly beused to measure oxygen concentration. These instruments are generallylarge assemblies with ionizing magnets and high vacuum pumps. Theadvantages of mass spectrometers include high accuracy, multi-gasanalysis capability, and rapid response. The disadvantages include highcost, high power consumption, and large size. Mass spectrometers are notsuitable for on-airway applications.

[0021] Raman scattering spectrometers (as described in U.S. Pat. No.4,784,486) can also be used to measure oxygen concentration. Thesedevices respond to photons emitted by the collision of a photon with anoxygen molecule. A photon from a high-power laser loses energy to theoxygen molecule and is re-emitted at a lower energy and frequency. Thenumber of photons re-emitted at the oxygen scattering wavelength isproportional to the number of oxygen molecules present. Like massspectrometers, Raman spectrometers have multi-gas analysis capabilityand rapid response time. Disadvantages include large size and powerconsumption. Therefore, Raman scattering photometers are not suitablefor on-airway applications.

[0022] Visible light absorption spectrometers (as described in U.S. Pat.Nos. 5,625,189 and 5,570,697) utilize semiconductor lasers that emitlight near 760 nm, an area of the spectrum comprised of weak absorptionlines for oxygen. With sophisticated circuitry, the laser can bethermally and/or electronically tuned to the appropriate absorptionbands. The amount of energy absorbed is proportional to the number ofoxygen molecules present. The advantages of this system are precision,fast response, and no consumable or moving parts. The disadvantagesinclude somewhat fragile optical components, sensitivity to ambienttemperature shifts, and a long gas sample path length. While there havebeen attempts to utilize this technology in an on-airway configuration,no commercially viable instruments have so far been available.

[0023] Luminescence quenching has also been proposed as a technique formeasuring oxygen concentration. In this approach a sensor contacted bythe gases being monitored is excited into luminescence. Thisluminescence is quenched by the oxygen in the monitored gases. The rateof quenching is related to the partial pressure of oxygen in themonitored gases, and that parameter can accordingly be used to providean indication of the oxygen in the monitored gases. However, the priorart does not disclose an oxygen concentration monitor employingluminescence quenching which addresses the problems associated with thistype of measurement device in any practical application. These problemsinclude: photo-degradation-associated and other instabilities of thesensor, low signal level, noise leading to difficulties in assessing thedecay of sensor luminescence, acceptably fast response times, thermaldrift of the sensor, reproducibility of the sensors, inaccuraciesattributable to stray light reaching the data photodetector, and theneed for light weight, ruggedness, and low power consumption. Disclosedin copending application Ser. Nos. 09/128,918 and 09/128,897, both filedAug. 4, 1998, are devices for monitoring oxygen concentration in gaseousmixtures which differ from the majority of the oxygen monitors describedabove in that they are compact, lightweight, and otherwise suited foron-airway mainstream monitoring of the oxygen concentration in aperson's respiratory gases. These monitoring devices utilize the fast(or breath-by-breath) approach to oxygen concentration monitoring withthe quenching of a luminescent dye being used in determining theconcentration of oxygen in the gases being monitored.

[0024] Fast (breath-by-breath) monitoring of end tidal oxygen is animportant diagnostic tool because, as examples only:

[0025] 1. It is a sensitive indicator of hypoventilation.

[0026] 2. It aids in rapid diagnosis of anesthetic/ventilation mishapssuch as (a) inappropriate gas concentration, (b) apnea, and (c)breathing apparatus disconnects.

[0027] 3. End tidal oxygen analysis reflects arterial oxygenconcentration.

[0028] 4. Inspired-expired oxygen concentration differences reflectadequacy of alveolar ventilation. This is useful for patients undergoingECMO (Extracaporeal Membrane Oxygenation) or nitric oxide therapies.

[0029] 5. When combined with a volume flow device (e.g. a pneumotach),VO₂ (oxygen consumption) can be determined. Oxygen consumption is a veryuseful parameter in determining (a) oxygen uptake during ventilation orexercise, (b) respiratory exchange ratio or RQ (respiratory quotient)and (c) general patient metabolic status.

[0030] The novel sensor devices disclosed in the copending applicationslocate a luminescent chemical in the patient airway. Modulated visiblelight excites the chemical and causes it to luminesce. The lifetime ofthe luminescence is proportional to the amount of oxygen present. Atransducer containing a photodetector and associated electroniccircuitry measures decay time and relates the measured parameter to theambient oxygen partial pressure.

[0031] The transducer device is small (<1 cubic inch), lightweight (lessthan 1 ounce), and does not contain moving parts. It utilizes visiblelight optoelectronics and consumes minimal power (system power less than2 watts). The unit warms up in less than 30 seconds, which isadvantageous in on-airway applications because of the need to takeprompt remedial action if a change occurs in a patient's conditionreflected in a change in respiratory oxygen concentration. The assemblydoes not require any significant optical alignment and is very rugged(capable of being dropped from 6 feet without affecting opticalalignment or otherwise damaging the device).

[0032] The principles of the inventions disclosed in the copendingapplications can be employed to advantage in sidestream (sampling) typesystems as well as in mainstream systems. This is important because somegas analysis systems, such as anesthetic analyzers, employ sidestreamtechniques to acquire their gas sample.

[0033] A typical transducer unit is easy to calibrate, stable (±2 torrover 8 hours at a 21 percent oxygen concentration), and has a highresolution (0.1 torr) and a wide measurement range (oxygenconcentrations of 0 to 100 percent). Response to changing oxygenconcentrations is fast (<100 ms for oxygen concentrations of 10-90percent at flow rates≈1|/min). The transducer is not susceptible tointerference from anesthetic agents, water vapor, nitrous oxide, carbondioxide, or other gases and vapors apt to be present in the environmentin which the system is used.

[0034] The sensor comprises a polymeric membrane in which a luminescablecomposition such as a porphyrin dye is dispersed. The sensor membrane isthe mediator that brings about dye-oxygen interaction in a controlledfashion. In a functional sensor, the dye is dispersed in the polymericmembrane, and oxygen diffuses through the polymer. The characteristicsof the sensor are dependent upon the dye-polymer interaction andpermeability and the solubility of oxygen in the polymer. Suchcharacteristics include the sensitivity of response of the sensor tooxygen, the response time of the sensor to a change in oxygenconcentration, and the measured values of phosphorescence intensity anddecay time. Thus the composition and molecular weight of the polymerdetermines the sensor characteristics. Also, if the sensor is preparedby evaporation of a solution as described in the copending applications,the film characteristics depend on the solvent that is used andconditions during casting or evaporation. If the dye is separately dopedinto the film from another solution, the solvent and conditions in thedoping medium also affect the sensor characteristics. When the polymerfilm is prepared by polymerization of a monomer or mixture, the sensorcharacteristics depend on the conditions of polymerization and suchresultant polymer characteristics as degree of crosslinking andmolecular weight.

[0035] The luminescent chemical sensor is not toxic to the patient andis a part of a consumable (i.e., disposable) airway adapter weighingless than 0.5 ounce. The sensor shelf life is greater than one year andthe operational life exceeds 100 hours. The cost of the consumableairway adapter is minimal.

[0036] It is also important that the oxygen monitoring systems disclosedin the copending applications have sufficient accuracy (1.0%), precision(0.01%), and response time (<100 ms) to monitor breath-by-breath oxygenconcentrations. The sensor is not sensitive to other gases found in theairway, including anesthetic agents, and is accordingly not excited intoluminescence by those gases. The sensitivity of the sensor totemperature, flow rate, pressure and humidity change is well understood;and algorithms which provide compensation for any errors due to thesechanges are incorporated in the signal processing circuits of thedevice.

[0037] The visible light oxygen measurement transducers disclosed in thecopending applications employ a sensor heater arrangement and aproportional-integrated-differential (PID) heater control system forkeeping the oxygen concentration sensor of the transducer precisely at aselected operating temperature. This is particularly significant becausethose oxygen measurement transducers employ a sensor which involves theuse of the diffusion of oxygen into a luminescable layer in measuringoxygen concentration. The rate of diffusion is temperature dependent. Asa consequence, the measurement of oxygen concentration becomesinaccurate unless the sensor temperature is kept constant. Also, if thewindow through which the excitation energy passes is not kept warm, itmay fog over. This also effects the accuracy of the oxygen concentrationmeasurement.

[0038] The location of the oxygen concentration sensor in a replaceable,simple component is a feature of the systems disclosed in the copendingapplications. This makes it possible to readily and inexpensively ensurethat the system is sterile with respect to each patient being monitoredby replacing the airway adapter between patients, avoiding thenon-desirability (and perhaps the inability) to sterilize that systemcomponent.

[0039] The provision of an airway adapter sensor and a separatesignal-producing transducer also has the practical advantage that ameasurement of oxygen concentration can be made without interruptingeither the ventilation of a patient, or any other procedure involvingthe use of the airway circuit. This is affected by installing the airwayadapter in the airway circuit. When the time comes to make oxygenmeasurements, all that is required is the transducer be coupled to theairway adapter already in place.

[0040] Another important feature of the invention insures that theairway adapter and transducer are assembled in the correct orientationand that the airway adapter and transducer are securely assembled untildeliberately separated by the system user.

[0041] The signals generated by the oxygen-measurement transducers ofthe previously disclosed system are processed to remove noise andextract the luminescence decay time, which is the oxygen-sensitiveparameter of interest. A lock-in amplifier is preferably employed forthis purpose. The lock-in amplifier outputs a signal which has a phaseangle corresponding to the decay time of the excited, luminescentcomposition in the oxygen concentration sensor. The lock-in detectioncircuitry rejects noise and those components of thephotodetector-generated signal which are not indicative of oxygenconcentration. This noise reduction also allows a higher level of signalgain which, in turn, makes possible enhanced measurement precision whiledecreasing the level of the visible excitation. This reduces instabilityfrom photoaging of the sensor, increasing accuracy and useable life. Allof this processing, which can be done with a digital, analog, or hybridmethod, is fast enough for even the most demanding applications such asthose requiring the breath-by-breath monitoring of a human patient.Various pathological conditions result in a change of oxygen demand bythe body. If a decrease of oxygen utilization by the body, for example,can be detected on a breath-by-breath basis, timely and effectiveremedial steps can be taken to assist the patient.

[0042] In the novel oxygen measurement transducers of the presentinvention, the concentration of oxygen in the gases being monitored isreflected in the quenching of an excited luminescent composition in theoxygen concentration sensor by oxygen diffusing into the sensor matrix.A source consisting of a light-emitting diode (LED) produces visibleexciting light which strikes the surface of the sensor film. Some of thelight is absorbed by the luminescent chemical dye in the film whereuponit produces luminescent light at a second, shifted wavelength. Thislight is captured by a photodetector which thereupon generates a signalreflecting the intensity and decay pattern of the intercepted light. Alllight directed toward the photodetector can potentially result in asignal. A suitable optical filter placed over the surface of thephotodetector discriminates against all but the luminescent light,thereby ensuring that the photodetector is producing a signal related tooxygen concentration only.

SUMMARY OF THE INVENTION

[0043] There have now been invented and disclosed herein new and noveloxygen concentration measuring devices which differ from those disclosedin the copending applications in that the light-sensitive, oxygenconcentration sensor is located on the same side of the gas samplingdevice (typically an airway adapter or a sampling cell) as the lightsource and detector of an associated transducer.

[0044] This “single-sided” arrangement of the light source, oxygensensor, and photodetector has a number of significant advantages.Specifically, in the systems disclosed in the copending applications,intimate contact between heater element components of the transducer andthe sampling device is required, and this can prove difficult toachieve. This problem is eliminated in the single-sided systemsdisclosed herein by supporting the sensor from a near side opticalwindow, and by heating that window which thereupon transfers thermalenergy to the associated sensor.

[0045] Another important advantage of the single-sided arrangementsdisclosed herein is that the energy of excitation indicative of oxygenconcentration does not have to traverse the gases flowing through thesampling component. Consequently, the degradation in signal attributableto interactions between the gas being sampled and the energy ofexcitation is eliminated, making a significantly less-degraded signalavailable to the photodetector.

[0046] One of the two apertures present in the sampling component of thepreviously disclosed systems is eliminated, along with a sensor filmheating component installed in that aperture. This leads directly to aless complex, less expensive sampling component. This is importantbecause the sensor film has a finite, relatively short life, and thesampling unit must accordingly be periodically replaced. In fact, in animportant application of the present invention —on-airway use in ahospital—it is highly desirable that the cost of the sampling unit below enough to make it feasible to discard this unit after a single use.

[0047] The location of the sensor film on the opposite side of a flowpassage from an optical window in the previously disclosed systemsleaves the optical window essentially unheated, making it particularlyprone to fogging. Contamination of this window may also be a problem,creating obstructions in the optical path between the sensor and thewindow.

[0048] The single-sided arrangement also makes feasible systemsembodying the principles of the present invention where it is desirableto have a unit such as a free-standing film reader in close proximity tothe sensor film as can be done with fiber-optics, for example. Sucharrangements can beneficially be used in sensor film quality control andin transcutaneous oxygen monitoring, for example. Such arrangements aremade practical by employing the principles of the present inventionbecause the sensor film is associated with the optical window and notisolated from the exterior of the sampling component by a thermalcomponent as disclosed in the copending applications.

[0049] Systems with the advantages just described differ physically fromthose disclosed in the copending applications in that the optical windowin the airway adapter or sampling cell is employed as a mount or supportfor the sensor film and is also employed to transfer to the film theheat needed to keep it at a constant temperature. As will be apparent,this also results in the window being heated to a high enoughtemperature to eliminate fogging. Various schemes for heating thetransparent window may be employed. One suitable approach is to surroundthe transparent window of the gas sampling device with a heater in aring configuration. Of importance in systems employing the principles ofthe present invention is a secure application of the film type sensor tothe optical window of the sampling device. An adhesive layer may beemployed to bond the sensor film to the window, or it may be solventbonded to the window. Another approach is to employ a retaining ring tostretch the film over and secure it to the window. A related approach isto employ a retaining ring bounded on one side with a fine mesh toretain the film and press it against the window. The last-mentionedapproach has the advantage that the film is physically retained withoutan adhesive and will not loosen. In addition, the mesh, with itslocation on the gas side of the sensor, enhances heat conduction overthat side of the sensor, producing exceptional thermal stability.

[0050] In monitoring apparatus embodying the principles of the presentinvention, light not indicative of the concentration of oxygen in thegas being monitored is preferably kept from the detector of thatapparatus by locating a blue dicorp filter and an infrared blockingfilter in line with and on the output side of the light source and bysimilarly locating a red dicorp filter and a red glass filter in frontof the detector apparatus. Because this arrangement eliminatesessentially all of the light which is not part of the oxygenconcentration indicative signal, the light collection efficiency isincreased to the extent that the intensity of the exciting light fromthe LED or other source can be reduced. This is important becausereducing the intensity of the light from the source significantlyincreases the service life of the sensor. This is particularlysignificant in sidestream applications of the present invention wherethe sensor is not apt to be replaced each time it is used.

[0051] Other objects, advantages, and features of the present inventionwill be apparent to the reader from the foregoing and the appendedclaims, and as the ensuing detailed description and discussion is readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In the drawings, like reference numerals refer to like partsthroughout the various views:

[0053]FIG. 1 is a block diagram of several main elements of aluminescence quenching gas detection apparatus of the present inventionshowing general interrelationships between the main elements.

[0054]FIG. 2 is a graph that qualitatively depicts characteristicemission curves of an excited sensing film of the present inventionshowing qualitative emission intensity vs. time.

[0055]FIG. 3 is a perspective view of an airway adapter and acomplementary transducer that shows a particular physical embodiment ofa portion of the block diagram of FIG. 1.

[0056]FIG. 4 is a generally pictorial view of an inline system formonitoring the oxygen concentration in a patient's breath.

[0057]FIG. 5 is a perspective view of an alternative airway adapter anda complementary transducer that shows the relationship of an opticalblock assembly to the airway adapter.

[0058]FIG. 6 is a diagram showing the optical alignment of key opticalcomponents of a transducer and sampling cuvette of the present inventionin a “single-sided” arrangement.

[0059]FIGS. 7 and 8 are diagrams showing the relationship of the opticalcomponents in “straight-through” and “two-sided” arrangementsrespectively, as disclosed in a prior copending application.

[0060]FIG. 9 is a cross-sectional view of the airway adapter andtransducer assembly of FIG. 5 showing the spatial relationship of keyoptical components in the optical block assembly.

[0061]FIG. 10 shows a perspective view of a side-stream embodiment ofthe present invention.

[0062]FIG. 11 illustrates a nasal canula component for sampling apatient's respiratory gases for subsequent monitoring by a side-streammonitor such as that shown in FIG. 10.

[0063]FIG. 12 depicts an exploded view of the side-stream embodimentshown in perspective in FIG. 10 showing pertinent details of deviceassembly.

[0064]FIG. 13 is a cross-sectional view of the side-stream gasmeasurement system of FIGS. 10 and 12 showing especially details ofoptical alignment and heater-to-sensing film relationship.

[0065]FIG. 14 is a block diagram of a DSP-based controller adapted for amainstream embodiment of the invention.

[0066]FIG. 15 is a block diagram that illustrates the methodology fordetermining oxygen concentration from the luminescence characteristicsof a sensing film.

[0067]FIG. 16 is a block diagram of a DSP-based controller adapted for asidestream embodiment of the invention, showing especially functionalityof the transducer-cuvette assembly.

[0068]FIG. 17 is a block diagram of a controller for a side-stream gasmeasurement system, showing especially functionality of the DSPcontroller, with correction for pressure and various output interfaces.

DETAILED DESCRIPTION OF THE INVENTION

[0069] The descriptions contained herein adhere to a numberingconvention intended to facilitate understanding and make for easycross-referencing of described features between figures. In thisconvention, the first digit (for features indicated by a three-digitreference number) or the first two digits (for features indicated by afour-digit reference number) correspond(s) to the figure number in whichthe feature is first described. Like features are thus identified by thesame reference number throughout the detailed description. In someinstances, features described by the same reference number may have adifferent physical appearance in two or more figures. In this case, theuse of a like reference number is especially useful in drawing theattention of the reader to various physical embodiments that a givenfeature of the invention may have. Features first introduced within thesame figure are numbered more-or-less consecutively in a mannercorresponding to the order in which they are described.

[0070] In each instance, physical forms depicted herein are intended tobe illustrative of particular embodiments of the invention. They aregiven such particular physical form to facilitate understanding. In nocase is the choice of a particular physical form intended to be limitingunless specifically so stated. A reader skilled in the art will readilyrecognize many alternative but equivalent physical embodiments, each ofwhich is intended to fall within the scope of the invention taughtherein.

[0071] Referring now to the figures, and in particular to FIG. 1 thereis illustrated a block diagram showing the main components andrelationships therebetween of a luminescence quenching oxygenconcentration monitoring apparatus in accordance with the principles ofthe present invention. A cuvette or airway adapter body 101 contains avolume 102 that serves as a gas sampling cell. For applicationsrequiring side-stream sampling of respiration or other gases, inletports 103 a and 103 b provide means for introduction of the gas to thesampling volume 102 and venting of gas from the sampling volume,respectively. For main-stream applications and other applicationsrequiring bidirectional transmission of the gas through the samplingvolume 102, the role of inlet ports 103 a and 103 b alternate withrespect to the instantaneous direction of gas passage therethrough. Asensing film 104 held within the sampling cell provides a medium for aluminescence quenching reaction that forms the basis of the measurementtechnique of the present invention.

[0072] A transducer 105 is closely coupled to the cuvette 101 so as toallow a light source 106 to illuminate the sensing film 104 withelectromagnetic radiation. The light energy emitted from light source106 is illustrated as a wavy line 107. For many applications, it isdesirable for the sampling volume 102 to be isolated from the transducer105. In these cases, an aperture 108 may take the form of a window setinto the housing of airway adapter 101 or may be formed integrallytherein.

[0073] According to the reaction used for gas measurement, excitationenergy 107 causes the sensing film to emit a luminescence, indicated bywavy lines 109, in a substantially omnidirectional manner at awavelength different from that of the excitation illumination 107. Theemitted luminescence 109 falls on a photo-detector 110 for measurement.The intensity and persistence of this luminescence rises and fallsaccording to the concentration of one or more gas components containedwithin the sampling volume 102. In a preferred embodiment of the presentinvention, oxygen causes a modification of the intensity and persistenceof the luminescent energy by quenching the luminescence reaction as itsconcentration increases. Thus, the luminescence quenching reaction isused to measure the amount of oxygen available to reaction sites withinthe sensing film 104. The quantity of oxygen available to the reactionsites may, in turn, be related to its partial pressure or concentrationwithin the measured gas.

[0074] According to a preferred embodiment of the present invention,light source 106, which may be in the form of a blue or green lightemitting diode (LED), is pulsed so as to provide excitation energy 107to the sensing film 104 that varies in time. Accordingly, luminescentenergy 109 emitted from the film varies in time at a substantially redwavelength. The photo-detector 110 in turn senses a cyclical variationin emitted energy, the persistence and intensity of which isproportional to the oxygen concentration of the gas introduced into thesampling volume 102 of the airway adapter 101. The inventors havediscovered that for many applications, the persistence of the emittedenergy 109 forms a more reliable and repeatable basis for measurement ofoxygen concentration than does the intensity or amplitude of the emittedenergy.

[0075] Transducer 105 is connected to control and measurement circuitry112 by means of electrical connections indicated by the line 111 .Control and measurement circuitry 112 may, in turn, be connected to anexternal computer, communication, display or other device by means ofconnections 113.

[0076] A temperature regulation apparatus 114, which, in a preferredembodiment, is a heater held in intimate contact with the sensing film104, is maintained in a relationship to the sensing film to provideadequate control of film temperature while not interfering with lighttransmission paths 107 and 109. As will be appreciated by the followingdiscussion, control of sensing film temperature is important toluminescence quenching rate as a function of oxygen concentration.

[0077] Taken together, the components of the block diagram illustratedin FIG. 1 form an oxygen concentration monitoring apparatus 115.

[0078] Turning our attention now to FIG. 2 there is illustrated aqualitative graph showing the relationship of the intensity andpersistence of luminescence in the sensing film as they may vary withoxygen concentration. The vertical axis is an arbitrary indication ofintensity or brightness of the luminescence, while the horizontal axisis an arbitrary indication of time. While no units are given in theillustration, the total timescale of the horizontal axis is generallywell under 1 second. For purposes of understanding FIG. 2, one mayassume that excitation energy begins to illuminate the sensing film att₀ and ceases at t₁. Curve 201 indicates the natural luminescence of thesensing film in the absence of oxygen. Higher concentrations of oxygenprogressively decrease both the peak luminescence and the luminescencedecay time. Curve 202 illustrates the effect of luminescence quenchingin the presence of moderate oxygen concentration of, for example, 21% at1 atmosphere pressure. Curve 203 shows a higher degree of luminescencequenching caused by a higher oxygen concentration, for example, of 50%at 1 atmosphere pressure.

[0079] By inspection of FIG. 2, one can see that both the peak luminanceand the decay time decrease as oxygen concentration increases. Bymeasuring the decay time over a series of excitation pulses, real-timemeasurement of oxygen concentration is effected.

[0080] It is of particular note that characteristic luminescenceresponse of the sensing film 104 as a function of oxygen concentrationis a strong function of film temperature. This is due to the fact thatit is the presence of oxygen within the sensing film at the site of eachluminescence reaction that determines whether or not that particularluminescence reaction will be quenched. In this manner, it is thestatistical proximity of oxygen molecules to the population ofluminescence reaction sites within the sensing film that determines theoverall macroscopic luminescence quenching effect illustrated by curves201, 202, and 203. The presence and concentration of oxygen within thesensing film 104 is a function of the rate of diffusion of oxygen withinthe film. As with most or all diffusion rate-limited reactions, oxygenluminescence quenching is thus a strong function of temperature.Accordingly, embodiments of temperature regulation apparatus play asignificant role in the enablement of the present invention.

[0081] Referring now to FIG. 3 there is illustrated a perspective viewof an embodiment of certain parts of the present invention wherein thesampling cell is in the form of a main-stream airway adapter. The airwayadapter 101 includes inlet/outlet ports 103 a and 103 b respectively.Aperture 108 is indicated by dashed lines and lies on an unseen side ofthe airway adapter. A transducer 105 is formed to securely attach to theairway adapter 101 by a snap fit, for instance. By forming the samplingcell and transducer in separate couplable bodies, the airway adapter 101may be readily made replaceable or even disposable without incurring theextra cost of replacing all of the optical and signal conditioningcomponents every time an airway adapter is discarded. It is particularlyadvantageous to form the sampling cell as a disposable unit formainstream applications so that each patient can be provided with his orher personal airway adapter without fear of contamination by anotherindividual. Making the airway adapter replaceable also serves to makeconnection of oxygen monitoring apparatus quick and easy and allows themore expensive transducers to be easily shared among multiple patientswithout causing an interruption in airway flow while removing orinserting a measuring apparatus. Finally, making the mainstream airwayadapter disposable also ensures that fresh sensing films are provided toeach patient. This is important due to a tendency for the sensing filmto gradually undergo photo-degradation.

[0082] The mainstream airway adapter 101 may be comprised of any of anumber of suitable materials. In one embodiment, airway adapter 101 is aone-piece unit typically molded from Valox polycarbonate or a comparablepolymer that is rugged and can be molded to close tolerances. An opaquematerial is employed to keep ambient light from reaching the sensor film104 through the walls of the airway adapter. Such extraneous light wouldadversely affect the accuracy of the oxygen concentration measurementsthat the system is designed to provide, or at least degrade thesignal-to-noise ratio of the characteristic signal, thus requiring moresophisticated and expensive control and detection means.

[0083] Airway adapter 101 has a generally parallelpipedal center section301 and hollow, cylindrical end sections 103 a and 103 b. Axiallyaligned passages 302 a, 102, and 302 b found in airway adapter elements103 a, 301, and 103 b, respectively, define a flow passage extendingfrom end-to-end of airway adapter 101. End section 103 a may beconfigured as a female connector and end section 103 b may be configuredas a male connector, thus allowing the airway adapter to be connected toconventional anesthetic and respiratory circuits.

[0084] The center section 301 of the airway adapter 101 is formed so asto fit snugly into a correspondingly shaped section 303 of transducer105. When airway adapter 101 is properly snapped into transducer 105,aperture 108 in the airway adapter is held in an orientation relative toa corresponding aperture 304 so as to allow passage of lighttherebetween. As described and shown in FIG. 1, excitation energy 107(see FIG. 1) comprised of blue or green light is transmitted fromtransducer 105, through apertures 304 and 108, and into contact with asensing film 104 (see FIG. 1) held in intimate contact with the gascontained within sensing volume 102. In response, and with a signalstrength and duration characteristic of the oxygen concentration of thegas in sensing volume 102, the sensing film 104 emits electromagneticradiation back through apertures 108 and 304 onto a detection element110 (see FIG. 1) held inside transducer 105 with a field of viewcomprising at least a portion of the sensing film 104 (through apertures304 and 108). In a preferred embodiment, apertures 108 and 304 containwindows which permit the transmission of both excitation andluminescence radiation therethrough.

[0085] Incorrect assembly of the airway adapter 101 into transducer 105is precluded by the inclusion of location features such as stops 305 and306 on the airway adapter and complementary stops 307 and 308,respectively, on the transducer.

[0086]FIG. 4 depicts an oxygen concentration monitoring apparatus 115 asit may be used in operation. A mainstream airway adapter 101 andtransducer 105, as illustrated in FIG. 3, make up the major componentsof inline assembly 401. The monitoring system 115 illustrated in FIG. 4also includes a hand held control and display unit 112 that is connectedto transducer 105 by a conventional electrical cable 111.

[0087] In the particular application of the present inventionillustrated in FIG. 4, system 115 is employed to monitor theconcentration of oxygen in a patient's respiratory gases. To this end,airway adapter 101 is connected in line between an endotracheal tube 402inserted in the patient's trachea and the plumbing 403 of a mechanicalventilator (not shown).

[0088] Airway adapter 101 and transducer 105 cooperate to produce anelectrical signal indicative of the oxygen concentration in the gasesflowing from endotracheal tube 402 through airway adapter 101 toplumbing 403. This signal is transmitted to unit 112 through cable 111and converted to a numerical designation that appears on the displayarray 404 of unit 112.

[0089] The two-component system 401 just described meets the requirementthat monitoring be accomplished without interrupting the flow of gasesthrough breathing circuit 403 or other patient-connected flow circuit.Transducer 105 can be removed—for example, to facilitate or enable themovement of a patient—leaving airway adapter 101 in place to continuethe vital flow of gases.

[0090] System 115 has, in this regard, the advantage that there are noelectrical components in the airway adapter. Hence, there are nopotentially dangerous electrical connections to the airway adapter whichmight expose the patient to an electrical shock.

[0091]FIG. 5 illustrates another embodiment of two-piece assembly 401.Airway adapter 101 includes the three sections 103 a, 301, and 103 bthat together form an inline gas flow passage 302 a, 102, and 302 b.Center section 301 of inline airway adapter 101 is formed to fit snuglyinto corresponding slot 303 of transducer assembly 105. Stops 305 and306 on airway adapter 101 are formed so as to create a snug fit withcorresponding stops 307 and 308 respectively; when inline airway adapter101 is coupled to transducer 105. Aperture 108, formed in a side ofairway adapter center section 301, contains a window 501. Window 501supports sensing film 104 (not shown) within sensing volume 102 andprovides a thermal energy transmission path from a temperatureregulation apparatus 114 (see FIG. 1) housed within transducer 105.

[0092] Transducer 105 contains an optical block assembly 502. Opticalblock assembly 502 contains the light source 106 and detector 110 (seeFIG. 1) in proper alignment. Optical block assembly 502 also houses aheater assembly 114 (not shown) for maintaining a constant temperaturewithin sensing film 104 (not shown). The use of an optical block 502 asa subassembly aids in the manufacturability of the transducer 105. Byincorporating all critical alignments and tolerances associated withtransducer 105 within optical block assembly 502, the manufacturingtolerances of the outer housing of transducer 105 may be loosenedsomewhat, thus reducing cost. Furthermore, service related to failure ofone or more components within the optical block 502 may be treated as asubassembly level repair, rather than forcing a replacement of theentire transducer assembly 105.

[0093]FIG. 6 is a conceptual diagram of the main optical components ofan embodiment of the present invention. Light emitting diode (LED) 106emits blue or green light in response to an energization signaltransmitted via leads 601. The blue or green light passes throughdichroic filter 602 and infrared blocking filter 603. In the embodimentillustrated in FIG. 6, the light energy then passes through an aperturein heater 114, through window 501, and falls upon sensing film 104.Sensing film 104 is held in intimate contact with window 501 by any of anumber of methods, such as for example, adhesive or solvent bonding, orvia a retaining ring or mesh covering. This allows the film 104 tofreely contact the gas within sensing volume 102.

[0094] LEDs are known to generally emit a relatively broad range oflight wavelengths extending to some degree even into the infrared. Thedichroic filter 602 and infrared blocking filter 603 cooperate tosignificantly reduce wavelengths other than the narrow range ofwavelengths passed by the dichroic filter. The particular wavelengthchosen for passage by the dichroic filter 602 may be selected tocorrespond to the peak output of LED 106 and to a suitable energizationwavelength for the sensing film 104. In a preferred embodiment, thiswavelength is chosen to be in the blue range of the visibleelectromagnetic spectrum.

[0095] Energization light incident upon sensing film 104 causes the filmto begin to emit light of a different wavelength. The sensing film maybe comprised, for instance, of a microporous polycarbonate film having aplatinum-porphyrin dye contained therein as in a guest-host system. Themicroporosity of the film represents a novel approach in the preparationof films designed for the monitoring of gaseous oxygen concentrations.The preparation of the polymeric membrane is well known in the art ofmanufacturing microporous screens and will not be described in detailherein. Suffice it to say that the process involves two steps whereinthe polymer film is exposed to collimated, charged particles in anuclear reactor which pass through the polymer leaving behind sensitizedtracks which are then etched into uniform cylindrical pores. Theincorporation of the luminescent sensing material into the film is morefully described in copending U.S. patent application entitled OxygenMonitoring Methods and Apparatus having Ser. No. 09/128,897, herebyincorporated by reference.

[0096] In one embodiment of the present invention, the emissionwavelength of the sensing film 104 corresponds to light in the redportion of the visible electromagnetic spectrum. An LED 106 isrepeatedly pulsed at a frequency of 20 kilohertz with its outputenergization signal 107 rising and falling as a sinusoidal wave. Thiscauses a rise and fall in luminescence energy emitted from the sensingfilm 104 as a function of oxygen concentration in sensing volume 102.This effect of a single pulse is qualitatively illustrated in FIG. 2.

[0097] Luminescence emitted by sensing film 104 passes through window501, through an aperture in heater 114, through red dichroic filter 604,through red filter 605, and impinges upon photo-detector 110. Red filter605 may be comprised of a conventional glass or gel filter. Red dichroicfilter 604 and red filter 605 cooperate to virtually eliminate any lightemitted by LED 106 through dichroic filter 602 and IR blocking filter603 from reaching photo-detector 110. The geometric relationship ofemitter and detector field-of-views further serve to reduce the amountof excitation energy reaching photo-detector 110 arising, for instance,from specular reflection off a surface of window 501.

[0098] Heater 114 is maintained in intimate contact with window 501 soas to maximize the effectiveness of the energy conduction path fromheater 114 through window 501 into sensing film 104. Maintainingconstant temperature within sensing film 104 is advantageous for keepingthe relationship between oxygen concentration within sensing volume 102and the amount of luminescence quenching sensed by photo-detector 110constant. Window 501 is preferably comprised of a material havingrelatively high thermal conductivity and high transparency such assapphire, glass, quartz, polycarbonate, or other material apparent tothose skilled in the art. Window 501 should be constructed so as tomaximize transmission of excitation energy and especially to maximizetransmission of luminescence energy. The materials listed above alsoaccomplish this aim. Furthermore, it is advantageous to maintain thetemperature of the sensing film 104 and window 501 somewhat above thetemperature of the gas in sensing volume 102. This serves to avoidcondensation of vapors on the window which may otherwise obscure thewindow, and reduce the effectiveness of the sensing apparatus.

[0099] The arrangement of emitter, detector, filters, and sensing filmdescribed by FIG. 6 are particularly effective at maximizingsignal-to-noise ratio of the detection apparatus of the presentinvention. The arrangement of electrical components shown in FIG. 6 onone side of sensing volume 102 serves to reduce cost and improvereliability compared to other arrangements wherein electrical componentsare arrayed on opposing sides of sensing volume 102. FIGS. 7 and 8illustrate configurations of the optical components representative ofsuch arrangements and of those disclosed in co-pending application Ser.No. 09/128,918.

[0100] Turning our attention now to FIG. 9, a cross-sectional view of atwo component assembly is illustrated generally at 401 showingespecially the means for optical alignment of key components isillustrated. The arrangement of components correlates to the embodimentdepicted in FIG. 6 in accordance with the principles of the presentinvention. The center section 301 of inline airway adapter 101 is shownheld in place within transducer housing 105. Center section 301 of theinline airway adapter 101 is held in correct optical alignment withoptical block 502 by means of the close fit between stop features 306and 308 (not shown) and between the outer walls of airway adapter 101and the inner walls of the transducer 105, as illustrated by FIGS. 3 and5.

[0101] Optical block 502 is comprised of an optical block casing 901that holds key optical components in boresight alignment by means of twobores created therein, light source bore 902 and detector bore 903, eachof which is aligned to hold their respective components so as to createsubstantially coincident fields of view of sensing film 104. LED 106 andfilters 602 and 603 are held in LED mounting tube 904. LED mounting tube904 may be constructed of brass tubing or other appropriate material.LED mounting tube 904 is coupled to light source bore 902 and holds theLED and filters for illuminating the sensor film 104. LED 106 receives asignal via leads 601 from optical block circuit board 905. Optical blockcircuit board 905 further provides means for mounting detector 110 andholding it aligned with detector bore 903. Light emitted from sensingfilm 104 thus passes through window 501, traverses detector bore 903,passes through red dichroic filter 604 and red filter 605, and impingesupon photo-detector 110. In a preferred embodiment, photo-detector 110is comprised of a photodiode.

[0102] Heater 114 is shown in cross-section with its aperturetherethrough allowing passage of both excitation energy and luminescentemission. Parts of heater 114 peripheral to the aperture are held inintimate contact with window 501. Sensing film 104 is maintained inintimate contact with window 501 by optional porous member 906 or byother means as described previously. Porous member 906 may be comprisedof any material that allows free passage of the gas in sensing volume102 to sensing film 104 and has appropriate tensile strength and heatresistance properties. In practice it has been found that it isespecially advantageous for porous member 906 to be comprised of astainless steel screen. In this embodiment, heat conduction along thewires of stainless steel screen 906 aids in the control and maintenanceof the temperature of sensing film 104.

[0103]FIG. 10 shows a perspective view of a side-stream embodiment ofthe present invention. Circuit board 1001 supports an optical blockassembly 502. A sampling cuvette 101 containing a sampling volume 102and inlet/outlet ports 103 a and 103 b is affixed to the optical blockwith machine screws (not shown) or by other means known in the art.Optical block 502 also includes a light source bore 902 which containsLED 106. LED 106 is, in turn, connected to circuit board 1001 and thecircuit thereon by means of leads 601.

[0104] The cuvette 101 may be made from machined and anodized aluminumwith ports 103 a and 103 b press-fit therein. Optical block housing 901may similarly be constructed from machined and anodized aluminum.

[0105] Circuit board 1001 may contain all or part of the control andmeasurement circuitry in addition to providing a mounting point foroptical block assembly 502. In some embodiments, circuit board 1001 maybe mounted inside diagnostic equipment such as an anesthesia monitor andprovide an interface 113 (not shown) to such equipment.

[0106]FIG. 11 illustrates a nasal canula component which may be employedto sample a patient's respiratory gases for subsequent monitoring by aside-stream monitor such as that shown in FIG. 10. The nasal canula ofFIG. 11 is of the conventional type typically found in hospitals orother health care facilities. It includes tubing 1101 that fits over thehead of a patient 1102. An insert 1103 in the tubing features a pair ofprotruding tube-shaped members 1104 that fit into the patient'snostrils. The nasal canula is connected as by tubular fitting 1105 to aflexible Nafine drying tube 1106. The drying tube removes moisture fromgases exhaled by patient 1102, thereby eliminating errors that moisturemight cause. At the far end of the Nafine drying tube 1106 is the femalecomponent 1107 of a conventional Leur fitting. A male Leur fitting (notshown) may be connected to a gas sampling tube (not shown) andtransmitted to a side-stream oxygen sensing device such as that of FIG.10 by means of a pump (not shown) such as a peristaltic pump.

[0107]FIG. 12 shows an exploded view of the side-stream gas measurementdevice illustrated in FIG. 10. Photodiode 110 is mounted through holesin photodiode mounting block 1201 to circuit board 1001 and thusconnected into the circuit thereon. Photodiode mounting block 1201 isitself glued to the surface of circuit board 1001 in order to holdphotodiode 110 at the correct height in detector bore 903, which isformed in optical block body 901. Filters 604 and 605 are mounted intothe detector bore of optical block body 901 in the manner indicated.Optical block body 901 is affixed to circuit board 1001 using opticalblock mounting screws 1202 a and 1202 b which extend through holes incircuit board 1001 into tapped holes 1203 (only one hole, 1203 a, isshown for clarity) formed diagonally across detector bore 903 in opticalblock body 901. Optical block locating stops 1204 a and 1204 b (notshown) are located on the opposite diagonal of detector bore 903 tooptical block mounting screws 1202 a and 1202 b and extend into holesformed in circuit board 1001 for aiding the proper location of opticalblock body 901.

[0108] LED mounting tube 904 extends into light source bore 902 inoptical block body 901 and is held therein via a press fit, trappingdichroic filter 602 and infrared blocking filter 603 against a shoulderformed within the light source bore. An optional diffuser may beinserted between dichroic filter 602 and LED 106 for reducing hot spotsin the LED emission pattern. LED 106 is held inside LED mounting tube904 using a press fit, adhesive mounting, or any suitable alternativemounting method. LED leads 601 extend through an aperture 1205 formed incircuit board 1001 and are soldered to traces on the bottom of thecircuit board.

[0109] Cuvette 101 is coupled to optical block body 901 with gasmeasurement volume 102 registered on axis to detector bore 903 using twoscrews 1206 a and 1206 b extending through corresponding holes incuvette 101 formed diagonally to gas measurement volume 102. Screws 1206a and 1206 b couple into corresponding tapped holes 1207 a and 1207 b,respectively, formed in optical block body 901. Ports 103 a and 103 bare inserted into cuvette 101 and may be attached via screws, pressfitting, adhesive, or may be formed integrally into the cuvette body, ormay be held in place using other means apparent to one skilled in theart. Stops 1208 a and 1208 b formed in optical block body 901 extendinto corresponding holes 1209 a and 1209 b formed in cuvette 101 at anopposite diagonal to screws 1206 a and 1206 b relative to detector bore903 and sensing volume 102. Stops 1208 a and 1208 b and theircorresponding holes 1209 a and 1209 b aid in locating the cuvetterelative to the optical block body and are especially useful duringassembly. The cuvette body may be constructed of machined aluminum,machined stainless steel, die cast metal, molded plastic, or othersuitable material.

[0110] Porous member 906, sensing film 104, and window 501 arecaptivated on a shoulder formed circumferentially to gas sensing volume102 in cuvette 101. These may be affixed by press fit or may be affixedin place using silicone adhesive or other alternative means apparent tothose skilled in the art. Window 501 may be comprised of sapphire,glass, quartz, plastic or other material. Materials for window 501 maybe chosen for their combination of high transparency at excitation andemission wavelengths as well as high thermal conductivity and lowthermal mass. Heater 114 is urged into intimate contact with window 501by heater springs 1210 which extend into corresponding holes 1211 formedin optical block body 901. In one embodiment, heater 114 is a ceramicheater with integral thermister. The use of springs 1210 to hold heater114 against window 501 helps to eliminate point loading and/or tighttolerance requirements on heater 114 and the corresponding gap betweencuvette 101 and optical block body 901. For the case where heater 114 isformed of ceramic or other brittle material, this arrangement alsoserves to reduce heater breakage during assembly and during service. Inone embodiment, springs 1210 may be formed from silicone rubber.

[0111] Referring now to FIG. 13 which is a cross-sectional view of theside-stream gas measurement system of FIGS. 10 and 12. Detector bore 903in optical block body 901 has two shoulders 1301 and 1302 formedcircumferentially at the bottom of the bore. Shoulder 1301 serves as astop for locating the top of red dichroic filter 604. Shoulder 1302serves as a stop for locating the top of photodiode mounting block 1201.Photodiode 110 is supported on photodiode mounting block 1201 andpresses up against red filter 605. Red filter 605, in turn, pressesagainst the bottom of red dichroic filter 604 and urges it againstshoulder 1301 in detector bore 903. When circuit board 1001 is affixedto optical block body 901 using screws 1202 a (not shown) and 1202 b,photodiode mounting block 1201 is urged against shoulder 1302 indetector bore 903. Photodiode mounting block 1201 also presses theassembly comprising photodiode 110, red filter 605, and red dichroicfilter 604 against shoulder 1301 in the detector bore. In this way, whenoptical block body 901 is affixed to circuit board 1001, the entiredetector assembly is securely coupled to its correct location in theoptical block body.

[0112] Light source bore 902 has one shoulder 1303 formed therein forlocating the end of LED mounting tube 904. Shoulder 1303 furthermoreserves to locate the top of infrared blocking filter 603. When LEDmounting tube 904 is pressed into light source bore 902 of optical blockbody 901, it pushes against the bottom of dichroic filter 602, urging itup into its correct location above LED 106. The top of dichroic filter602, in turn, presses against the bottom of infrared blocking filter603, which itself is urged against shoulder 1303 in light source bore902. In this way, the proper insertion of LED mounting tube 904, withLED 106 held therein, in light source bore 902 captures the entire lightsource assembly comprising the LED, dichroic filter 602, and infraredblocking filter 603 at its correct position in optical block body 901.

[0113] LED mounting tube 904 and the rest of the light source assemblymay be inserted into the light source bore 902 of optical block body 901through aperture 1205 in circuit board 1001 after securely affixing theoptical block body to the circuit board using screws 1202 a and 1202 b.Alternatively, the light source assembly may be inserted into the lightsource bore 902 prior to attaching the optical block body 901 to circuitboard 1001. In either case, LED leads 601 may be subsequently bent intoposition contacting their corresponding electrical traces (not shown) oncircuit board 1001 and soldered thereto. Alternatively, other types ofsocketed connectors may be used to receive LED leads 601 or theirequivalent or other types of permanent connection may be made.

[0114] Cuvette body 101 has a shoulder 1305 formed circumferentially tothe bottom aperture of gas measurement volume 102. Shoulder 1305 servesas a location feature for locating the sensor and window assemblycomprising porous member 906, sensing film 104, and window 501 relativeto gas measurement volume 102. Optical block body 901 has a depressedplanar area 1304 corresponding to and extending beyond shoulder 1305formed between cuvette mounting surfaces. This serves to provide avolume for accepting heater 114 and any protruding thickness of window501. Four heater spring holes 1211 extend from planar area 1304 into thevolume of optical block body 901. Four heater springs 1210 are insertedinto heater spring holes 1211 prior to placing heater 114 thereon withits aperture located axially along detector bore 903. Cuvette 101 withthe sensor and window assembly seated therein is placed over heater 114and located with window 501 aligned axially to detector bore 903. Stops1208 a (see FIG. 12) and 1208 b formed in optical block body 901 extendinto holes 1209 a (see FIG. 12) and 1209 b, respectively, formed incuvette 101. Stops 1208 a and 1208 b and their corresponding holes 1209a and 1209 b aid in the alignment of window 501, sensing film 104,porous member 906, and gas sampling volume 102 to the detector bore 903formed in the optical block body during assembly and service. As cuvettemounting screws 1206 a and 1206 b are tightened, heater springs 1210compress in their bores 1211 and urge heater 114 against the bottom ofwindow 501. This upward pressure on window 501 further compressessensing film 104 and porous member 906 against shoulder 1305 in sensingvolume 102 of cuvette 101. As screws 1206 a and 1206 b are torqued topredetermined values, the bottom of cuvette 101 comes into closecoupling with the top surface of optical block body 901. Thus the use ofheater springs 1210 to compress the assembly comprising heater 114,window 501, sensing film 104, and porous member 906 against shoulder1305 causes the entire sensor and window assembly to be brought intocorrect optical alignment with other components of optical block 502when cuvette 101 is properly coupled against optical block body 901.

[0115]FIG. 14 is a block diagram of a controller for controlling the gasmeasurement apparatus of the present invention and for receiving datathat may be converted to gas concentration information. The controllerof FIG. 14 is particularly applicable to a mainstream gas analyzer suchas that depicted by FIGS. 3 through 5.

[0116] The main assemblies shown in FIG. 14 include a controllercorresponding to block 112 from FIG. 1, transducer 105, and cuvette orairway adapter 101 containing sensing film 104. Transducer 105 containsLED 106, photo-detector 110, and heater 114, and additionally athermostat 1401, a memory 1405, and a photo-detector pre-amp 1409.

[0117] Control and measurement lines 111 connect controller 112 totransducer 105 and include cuvette temperature signal 1402, heatercontrol line 1403, data line 1406, LED drive 1407, and oxygen signal1410. Excitation light 107, luminescence light 109, and heat conductionpath 1404 form the interface between transducer 105 and airway adapter101.

[0118] Digital Signal Processing (DSP) controller 112 may, for example,contain control and detection circuitry as well as communicationscircuitry and logic for communicating with a host computer and/or fordisplaying gas concentration measurement data to the user. One aspect ofsystem operation controlled by DSP controller 112 is the temperature ofthe sensing film 104.

[0119] Heater 114 may contain an integral thermostat 1401 or,alternatively, may contain a separate thermostat 1401. In any event,heater 114 may preferably contain a circuit to cut heater drive in theevent of heater control failure. Thermostat 1401 and associated heatercut-off circuit serves as a fail-safe device to avoid runaway heaterdrive and a resultant possible unsafe situation or destruction ofsensing film 104. Cuvette temperature is transmitted to the DSPcontroller circuit by an analog signal 1402 the voltage of which isproportional to the temperature of heater 114 and, by extension, thetemperature of sensing film 104. Analog cuvette temperature signal 1402may, for instance, be generated by a thermistor integral to or otherwisecoupled to heater 114 or, alternatively, coupled to a convenientlocation whose temperature varies proportionally to the temperature ofheater 114. Heater control signal 1403 is driven from DSP controller 112as a pulse width modulated (PWM) digital control signal whose duty cycleis controlled by a fuzzy logic controller embedded within DSP controller112. The fuzzy logic portion of the DSP controller is programmed in amanner similar to a proportionalintegral-differential (PID) controller.Fuzzy logic embedded in the DST controller 112 monitors the analogcuvette temperature signal 1402 via an analog-to-digital (A/D) converterand controls the duty cycle of PWM heater control signal 1403 inresponse. The duty cycle of heater control signal 1403 is controlled tobe higher when the cuvette temperature is cooler and duty cycle iscontrolled to be lower when the cuvette temperature is warmer. Inpractice, this control methodology may be used to maintain a constanttemperature in sensing film 104. Heater control signal 1403 drives atransistor (not shown) that may, for instance, be integral to heater114. The transistor driven by PWM heater control signal 1403 acts as arelay that switches drive current to heater 114 on or off. Heat flowsfrom heater 114 to sensing film 104 via a heat conductive path 1404. Bysetting the temperature of sensing film 104 above that of the flowinggas to be sensed, heat always flows from the heater 114 to the sensingfilm. The amount of heat modulated by heater control signal 1402 thusmay always act as a positive control signal, heat never needing to beremoved from the system.

[0120] Memory element 1405, which may, for instance, be embodied aselectrically erasable programmable read-only memory (EEPROM) or flashmemory, is associated with a transducer 105. Memory 1405 contains atransducer serial number and calibration information indicating oxygenconcentration vs. phase shift. At boot-up, controller 112 reads thetransducer serial number from memory 1405 to determine if propercalibration information has been loaded. If the transducer 105 is thesame unit that had been connected to controller 112 during its previousoperational session, no further data is read from memory 1405 andboot-up continues. If the serial number encoded within memory 1405indicates that transducer 105 is a new pairing with controller 112,calibration data and the serial number is read from memory 1405 andwritten in non-volatile form into memory (not shown) contained withincontroller 112. Upon subsequent boot-ups with the same transducer 105,this previously stored calibration data is used directly.

[0121] During operation, controller 112 drives emitter 106 with a phaseangle modulated signal via LED drive 1407. Light 107 emitted from LED106 is pulsed onto sensing film 104 with phase angle modulationcorresponding to the signal 1407. In a preferred embodiment, light 107emitted from LED 106 has a spectral distribution predominantly in theblue portion of the electromagnetic spectrum and serves to excitesensing film 104 into luminescence. Photo-detector 110 transformsluminescent energy 109 into a current- or voltage-modulated electricalsignal 1408 which, in turn, is amplified to a usable oxygen signal 1410by pre-amplifier 1409. Pre-amplifier 1409 may be, for instance, a lownoise operational amplifier. Oxygen signal 1410 is transmitted tocontroller 112 via a conventional conductive wire where it is used todetermine oxygen concentration within airway adapter 101.

[0122] The oxygen signal 1410 may be a function of several factors inaddition to oxygen concentration including pre-amp 1409 characteristics,photo-detector 110 characteristics, and other detector opticalidiosyncrasies. Luminescent light 109 emitted from sensing film 104 hasa temporal intensity curve (similar to curves shown in FIG. 2) relatedto excitation energy 107 received from LED 106, sensing filmtemperature, oxygen concentration within airway adapter 101, andpossibly the amount of previous photo-degradation of sensing film 104.The particular amount and quality of excitation energy 107 emitted byLED 106 varies according to LED output efficiency and spatialdistribution, variations in alignment and transmissivity of theparticular components of the transducer emitter assembly as well as thephase angle modulated signal input via LED drive 1407.

[0123] The effects of factors other than oxygen concentration and LEDdrive signal may, to a great extent, be eliminated, thus simplifying theproblem of determining concentration. Transducer-specific factors suchas pre-amp characteristics, detector assembly characteristics,variations in heater calibration, variations in overall LED outputefficiency, and other alignment variations may be eliminated fromconsideration by use of the transducer-specific calibration datacontained within memory 1405 according to the method described above.Variations in sensing film oxygen diffusivity (as a function oftemperature) may be eliminated by keeping the sensing film 104 at aconstant temperature using methods described above. Deleterious effectsdue to sensing film photo-degradation may be largely eliminated bypackaging the sensing film 104 as a part of a disposable airway adapter101, thus ensuring that the sensing film is always fresh. Thus, theproblem of determining oxygen concentration is simplified to comparingthe oxygen signal 1410 to the phase angle modulated LED drive signal1407.

[0124]FIG. 15 is a block diagram that describes more specifically theprocess of comparing the LED drive signal 1407 to the oxygen signal 1410to determine oxygen concentration. A portion of the DSP controller 112is shown with connections to the transducer 105 comprising an LED drive1407 and an oxygen signal 1410. The memory heater and thermostat, aswell as their corresponding connections are omitted from FIG. 15 for thesake of clarity. DSP integrated circuit 1520 forms the heart ofprocessing functionality and CODEC 1521 provides analog/digitalinterfaces on DSP controller 112. Current voltage converter 1409corresponds to pre-amp 1409 in FIG. 14 and is indicative of oneembodiment. As described in conjunction with FIG. 14, LED drive 1407pulses LED 106 which emits a corresponding excitation signal 107 toexcite luminescence in sensing film 104. Upon receiving a pulse ofexcitation energy 107, sensing film 104 emits luminescent energy 109with an intensity and duration inversely proportional to oxygenconcentration in the sampling volume 102 (not shown) of the airwayadapter 101, as shown by FIG. 2. Photodiode 110 converts variations inluminescence 109 to corresponding variations in current 1408 thatcurrent voltage converter 1409, in turn, amplifies and converts tovariations in voltage prior to transmitting the resultant oxygen signal1410 back to the DSP controller 112. Signals 109, 1408, and 1410 thusare effectively phase-retarded output signals with the amount of phaseretardation determined by oxygen concentration.

[0125] For the purposes of the signal processing to be done, transducer105 may be considered a trans-impedance amplifier. LED drive 1407 andreference channel 1501 are driven as pure sine waves. Due toperturbations introduced by sensing film 104, oxygen signal 1410 ismodified somewhat from the pure sine wave of LED drive 1407. Theperturbations introduced by sensing film 104 are, of course, the verysignal from which oxygen concentration may be derived.

[0126] Oxygen signal 1410 is passed to DSP controller 112 and sentthrough anti-aliasing filter 1502 to remove phase delays relative to LEDdrive 1407 introduced by propagation delays along the signal pathlength, thus producing anti-aliased oxygen signal 1503. Referencechannel 1501, nominally driven in quadrature to LED drive 1407, issimilarly passed through anti-aliasing filter 1504 to produceanti-aliased reference signal 1505.

[0127] Anti-aliased oxygen signal 1503 and anti-aliased reference signal1505 are converted to digital signals by passing throughanalog-to-digital (A/D) converter channels 1506 and 1507, respectively.Digital oxygen signal 1508 and digital reference signal 1509, whichresult from the A/D conversion, are then mixed by mixer 1510 to createAC coupled error signal 1511. Digital mixer 1510 multiplies signals 1508and 1509 point-by-point to produce signal 1511. AC coupled error signal1511 is subsequently processed by digital low pass filter 1512 to removethe AC coupling and produce DC error signal 1513. DC error signal 1513has a voltage proportional to the signal perturbations (phase delay)introduced by the luminescence quenching oxygen measurement film 104 inconverting LED drive signal 1407 to oxygen signal 1410. Less phase delayin the signal channel relative to the reference channel, correspondingto higher oxygen concentrations, results in a lower DC error signal1513. Conversely, greater phase delay in the signal channel relative tothe reference channel corresponds to lower oxygen concentration and ahigher DC error signal 1513.

[0128] Dual output variable phase drive 1514 outputs digital waveforms1515 and 1516 which are converted by digital-to-analog (D/A) converterchannels 1517 and 1518, respectively, to create LED drive 1407 andreference channel drive 1501, respectively. Frequency is held constantby drive 1514 while phase of the two channels is varied relative to oneanother. Specifically, drive 1514 advances the phase of digitalreference channel 1516 in response to DC error signal 1513 to minimizethe magnitude of DC error signal 1513. The amount of phase advance,indicated as N⁰, required to minimize the magnitude of DC error signal1513 is thus proportional to oxygen concentration. The value of N⁰ isoutput via digital output line 1519 for further processing andinterpretation, either by embedded processes or by a host computer.

[0129]FIG. 16 is a block diagram of controller components for aside-stream gas measurement transducer and cuvette such as the systemshown in FIGS. 10, 12, and 13 focusing especially on functionalityincorporated in transducer/cuvette assembly 401. FIG. 16 alsocorresponds relatively closely to FIG. 14 which is an implementationspecific to a mainstream gas measurement system.

[0130] The main difference between the block diagram of FIG. 16 and theblock diagram of FIG. 14, aside from the physical implementation, is theaddition of a pressure sensing transducer 1601 and corresponding dataline 1602 in the block diagram of FIG. 16. Because gases delivered toside-stream gas analysis systems are pumped to the sampling cuvette 101,there is a possibility of an overpressure situation in which the gaspressure within cuvette 101 is above atmospheric pressure. As wasdescribed in conjunction with FIG. 2, a higher sample gas pressure couldlead to mistaken calculation of a higher-than-actual oxygenconcentration.

[0131] The addition of pressure transducer 1601 yields two advantages.First, oxygen concentration calculated using an atmospheric pressureassumption may be corrected according to measured pressure to yieldactual oxygen concentration. Secondly, feedback control may be used tocontrol the pump (not shown) to reduce actual sample volume pressure toatmospheric pressure.

[0132] Other functionality of the block diagram of FIG. 16 is similar tocorresponding features shown and described in FIG. 14.

[0133]FIG. 17 is a block diagram of a side-stream gas measurementcontroller showing especially functionality incorporated in the DSPcontroller 112. Signals from transducer/cuvette assembly 401 are asshown and described in FIG. 16.

[0134] Analog-to-digital converter 1701 is configured as a multi-channeldevice, receiving analog input from various sensors and providingdigital representations of the analog signals to the DSP chip 1520 viadigital signal path 1704. Cuvette temperature 1402 is provided as a DCvoltage and converted by A/D converter 1701 into a digital form forprocessing by DSP chip 1520 which, in response, modulates PWM heatercontrol line 1403. An ambient pressure transducer 1702 is connected toA/D converter 1701 by analog line 1703 and the cuvette pressuretransducer 1601 (not shown) is connected to A/D converter 1701 by analogline 1602. These analog signals are converted to corresponding digitalsignals and transmitted to DSP chip 1520 via digital line 1704. Digitalline 1704 may, for instance, be configured as a multi-channel parallelinterface. By comparing the ambient pressure to cuvette pressuredifferential, DSP chip 1520 may, for instance, provide feedback toprocess computer 1705 to enable pump control. By measuring cuvettepressure, DSP chip 1520 may correct for errors in measured oxygenconcentration due to absolute pressure variations.

[0135] DSP controller 112 may communicate with process computer 1705 viaa serial data communications line 1706. Serial communications interface1706 may use, for instance, an RS-232 protocol. Communication interface1706 may utilize fixed commands by the process computer 1705 to controland calibrate DSP controller 112. In one embodiment, oxygenconcentration data is sent from DSP controller 112 to process computer1705 as a response to command by the process computer. In this way, theprocess computer only receives data when such data is needed and it isready to receive data.

[0136] CODEC 1521 receives an oxygen signal 1410 from the side-streamassembly 401, converts it into digital channel 1508, and transmitsdigital channel 1508 to DSP chip 1520 as shown and described in FIG. 15.CODEC provides an interface between digital input and output (I/0) ofDSP chip 1520 and various analog lines, only two of which are shown inFIG. 17 for clarity. Digital interface 1707 is actually a composite ofseveral digital channels including 1508, 1509, 1515, and 1516. CODEC1521 converts a digital LED drive signal 1515 (not shown) into acorresponding LED analog signal 1708. LED analog signal 1708 is thenamplified by LED driver 1709 and sent to sidestream assembly 401 via LEDdrive 1407 to drive LED 106 (not shown).

[0137] EEPROM data line 1406 operates as shown and described in FIGS. 14and 16.

[0138] Digital output line 1519 is converted to an analog signal 1711 bydigital-to-analog converter (DAC) (elsewhere referred to as “D/Aconverter”) 1710. Analog line 1711 may be used, for instance, to driveanalog gauges or other devices for displaying oxygen concentration datato a user.

[0139] While the invention is described and illustrated here in thecontext of a limited number of preferred embodiments, the invention maybe embodied in many forms without departing from the spirit of theessential characteristics of the invention. The present embodiments aretherefore to be considered in all respects as illustrative and notrestrictive. The scope of the invention is indicated by the appendedclaims rather than by the foregoing description, and all changes whichcome within the meaning and range of equivalency of the claims areintended to be embraced therein.

What is claimed is:
 1. An oxygen concentration monitoring apparatuscomprising: a sensor which includes a luminescable composition; a lightsource which provides light of a first wavelength capable of excitingthe composition into luminescence; a flow component for so bringing agas to be monitored into contact with the excited, luminescingcomposition that the luminescence is quenched in a manner reflecting theconcentration of oxygen in the gas being monitored; and a detectorimpinged upon by light of a second wavelength emitted by the excited,luminescing composition for producing a signal indicative of theconcentration of oxygen in the gas; the flow component having a nearwall and a far wall bounding a sampling volume for the gas beingmonitored; the oxygen sensing sensor being located at the near wall ofthe flow component; and the light source and the detector being locatedoutside of the sampling volume adjacent the near wall.
 2. An oxygenconcentration monitoring apparatus as defined in claim 1 in which thelight source and the detector are components of a transducer which canbe detachably assembled to the flow component.
 3. An apparatus asdefined in claim 1 in which the flow component is an airway adapter. 4.An apparatus as defined in claim 1 in which the flow component is asampling cell.
 5. An oxygen concentration monitoring apparatus asdefined in claim 1 which comprises a heater for maintaining the sensorat a selected temperature.
 6. An oxygen concentration monitoringapparatus as defined in claim 1 which includes components for keepingfrom the detector extraneous visible light which is not indicative ofthe concentration of oxygen being monitored.
 7. An oxygen concentrationmonitoring apparatus as defined in claim 6 wherein the componentscomprise: a blue filter and an infrared blocking filter on the outputside of and in alignment with the light source; and a red dicorp filterand a red glass filter in front of the detector.
 8. An oxygenconcentration monitoring apparatus as defined in claim 1 in which: thesensor comprises a luminescable composition dispersed in a polymericmembrane; and the sensor is positioned for contact by the gas beingmonitored by a support made of a material which transmitselectromagnetic energy of the first and second wavelengths.
 9. An oxygenconcentration measuring apparatus which comprises: a sensor whichincludes a luminescable composition; a source of light for exciting thecomposition into luminescence; and a unitary airway adapter or samplingcell for so flowing a gas to be monitored in relation to the compositionthat the luminescence of the excited composition is quenched in a mannerindicative of the concentration of oxygen in the gas; and the airwayadapter or sampling cell having a flow passage for the gas and a windowopening onto the flow passage via which light from the source can reachthe luminescable composition.
 10. An apparatus as defined in claim 9:wherein the component for flowing gas into contact with the sensor is anairway adapter; the apparatus further comprising a transducer to whichthe airway adapter can be detachably coupled, the transducer comprisingsource of light, a detector sensitive to electromagnetic energy emittedfrom the luminescable composition for producing a signal which is afunction of the oxygen concentration in the gas flowing through theairway adapter, and a casing in which the source of light and thedetector are mounted.
 11. An oxygen concentration monitoring apparatusas defined in claim 9 in which the transducer further comprises aplatform housed in the casing, the light source and the detector beingmounted to and thereby integrated with the platform.
 12. An oxygenconcentration monitoring apparatus as defined in claim 9 in which thesource of light is an LED which emits electromagnetic energy in the300-600 nm band.
 13. An oxygen concentration monitoring apparatus asdefined in claim 12 in which the source of light is a blue or green LED.14. An oxygen concentration monitoring apparatus as defined in claim 9in which the luminescable composition is a phosphorescent organometalliccomplex having a 400-700 mn absorption band and a 500-1100 nm emissionband.
 15. An oxygen concentration monitoring apparatus as defined inclaim 14 in which the luminescable composition is a fluorinatedporphyrin.
 16. An oxygen concentration monitoring apparatus as definedin claim 15 in which the luminescable composition is selected from thegroup consisting of palladium meso-tetraphenyl porphine, platinummeso-tetraphenyl porphine, palladium meso-tetra (perfluoro) phenylporphine, and platinum meso-tetra (perfluoro) porphine.
 17. An oxygenconcentration monitoring apparatus as defined in claim 9 in which theluminescable composition is embedded in a matrix, the matrix being amicroporous hydrophobic polymer having a thickness in the range of 5 to20 μm and a pore size in the range of 0.10 to 10.0 μm.
 18. An oxygenconcentration monitoring apparatus as defined in claim 17 in which thepolymer is: a silicone, a polycarbonate, a polystyrene, a polymethylmethacrylate, a polyvinyl chloride, a polypropylene, a polyester, or anacrylic copolymer.
 19. An oxygen concentration monitoring apparatus asdefined in claim 18 in which the polymer is a track etchedpolycarbonate.
 20. An oxygen concentration monitoring apparatus whichcomprises: a sampling unit which has a wall bounding a sampling volume;an optical window in the wall; a sensor which comprises a luminescablecomposition distributed in a porous matrix, the sensor being supportedin the sampling volume for contact by gas therein by the window; a lightsource for transmitting through the window to the sensor energy of awavelength capable of exciting the composition into luminescence; and adetector for energy of excitation emitted by the sensor and transmittedthrough the window to the detector.
 21. An apparatus as defined in claim20 which comprises a heater unit for heating the window to a temperaturehigh enough to keep condensate from forming on the window.
 22. Anapparatus as defined in claim 20 which comprises a heater unit for soheating the window as to keep the sensor at a specified temperature bythe transfer of heat from the window to the sensor.
 23. An apparatus asdefined in claim 20: which includes a transducer comprising the lightsource, the detector, and a monolithic mount for the light source andthe detector.
 24. An apparatus as defined in claim 20 in which: there isan aperture in the near wall; a light transmitting window in theaperture; and the sensor is mounted to the window.
 25. An oxygenconcentration monitoring apparatus as defined in claim 20: wherein thedetector is responsive to energy in the visible light range; wherein theenergy emitted by the sensor and indicative of the concentration ofoxygen in the gases being monitored is in the visible light range; andwherein the apparatus includes means for keeping from the detectorextraneous visible light which is not indicative of the oxygenconcentration.
 26. An oxygen concentration monitoring apparatus asdefined in claim 25 in which the means for keeping extraneous visiblelight from the detector comprises a filter in an optical path betweenthe sensor and the detector.
 27. An oxygen concentration monitoringapparatus as defined in claim 9 which comprises means for processing thedetector generated signal into an oxygen concentration signal.
 28. Anoxygen concentration monitoring apparatus as defined in claim 9 in whichthe sensor comprises a luminescable composition embedded in a porouspolymer.
 29. An apparatus as defined in claim 9: wherein the samplingunit is a cell; the apparatus further comprising a pump for effecting aflow of the gas to be monitored to the cell.
 30. An apparatus as definedin claim 9 in which the sampling unit is an online airway adapter. 31.An oxygen concentration monitoring apparatus which comprises: a sensorwhich comprises a luminescable composition dispersed in a porous matrix;a light source for providing to the sensor light of a wavelength capableof exciting the composition into luminescence; an arrangement for sobringing a gas to be monitored into contact with the sensor that theexcited, luminescing composition is quenched in a manner reflecting theconcentration of oxygen in the gas being monitored; a detector forintercepting light emanating from the excited, luminescing sensorcomposition and for outputting a signal reflecting parameters of theintercepted light; and a signal processor for making a phase-sensitivemeasurement of the signal, the signal being indicative of theconcentration of oxygen in the gas being monitored.
 32. An apparatus asdefined in claim 31 which has a digital signal processor for the signaloutputted from the detector.
 33. A sampling device for an oxygenconcentration monitoring apparatus comprising: a casing defining avolume for a gas being monitored; an optical window mounted in anaperture in the casing; an oxygen sensor film comprising a compoundwhich can be excited into luminescence by light transmitted through thewindow to the sensor film; and a device for fixing the sensor film tothe window, the device comprising: a ring surrounding the window andmounting the window in the casing aperture; and a mesh overlying thesensor film and pressing the sensor against and into intimate contactwith the window.
 34. A replaceable airway adapter for oxygenconcentration monitoring apparatus, the airway adapter comprising: aflow passage-defining casing; a luminescable, oxygen quenchable sensorexposed to the flow passage; and an optical window mounting the sensorto the casing.
 35. A sampling device as defined in claim 33 whichcomprises: a windowed aperture on one side of the casing fortransmitting electromagnetic energy emitted by luminescence from thesensor; the sensor being located on the same side of the passage as thewindowed aperture.
 36. A sampling device as defined in claim 33 in whichthe sensor film comprises a luminescable composition embedded in aporous polymeric film.
 37. A sampling device as defined in claim 36 inwhich the polymeric film is stretched over the window and has an edgeportion trapped between the window and the edge of the aperture.
 38. Asampling device as defined in claim 33 in which the casing has integralmisalignment prevention means.
 39. A sampling device as defined in claim33 in which the casing has integral end sections configured to provideconnections for coupling the sampling device to components of an airwaycircuit.
 40. A sampling device as defined in claim 33 wherein the casingcomprises: a center section and two end sections; the end sections beingaligned with the center section defining a flow passage extendingseriatim through one of the end sections, the center section, and theother of the end sections.
 41. A sidestream sampling system formonitoring the concentration of gases flowing through the system, thesampling system comprising: a sampling cell comprising a sensor whichhas an oxygen sensitive, luminescable composition, the sampling cellbeing located in the system flow path; a complementary transducer whichincludes casing which houses a light source for exciting the compositioninto luminescence and a detector for intercepting energy emitted by theexcited, luminescing composition for producing a signal indicative ofthe concentration of oxygen in the gases; and a pump for so effecting aflow of the gases being monitored through the sampling cell and intocontact with the luminescing composition that the luminescence isquenched in a manner indicative of the concentration of oxygen in thegases; the sampling cell including a windowed aperture; the sensor beingin intimate contact with a first side of a window in the aperture; andthe light emitter and the detector being located by the transducercasing on a second, opposite side of and in close physical proximity tothe window.
 42. A system as defined in claim 41 in which: the samplingcell has a casing; the sampling cell casing has a flow passagetherethrough; and the system comprises a port providing communicationbetween the passage and the ambient surroundings to bring the system toatmospheric pressure and thereby establish a baseline pressure in thesystem.
 43. A system as defined in claim 41 which includes anarrangement active during the operation of the system to identifyocclusions in the system by comparing the actual pressure in the systemwith the baseline pressure.
 44. A system as defined in claim 42 inwhich: the casing provides a flow path from a source of gases to bemonitored through the sampling cell and to the pump; a motor for thepump; components for so controlling the speed of the motor as tomaintain a constant pressure in the system; and a differential pressuretransducer for providing to the motor speed control signals reflectingthe rate-of-flow in the system flow path; there being a flow restrictorin the flow path and means providing fluid communication between: (a)the flow path and the transducer, and (b) the flow restrictor and thetransducer.
 45. A system as defined in claim 41 which comprises: anabsolute pressure transducer; a differential pressure transducer; and avalve which is selectively operable to: (a) provide communicationbetween the system flow path and the absolute pressure transducer; (b)the flow path and the ambient surroundings; and (c) the flow path andthe differential pressure transducer.
 46. A system as defined in claim41 in which there is an accumulator in the system flow path to dampenthe pressure pulses generated by the operation of the system pump.
 47. Asystem as defined in claim 45 in which there is a flow restrictor in theflow path that further dampens pressure pulses attributable to thecyclic operation of the pump.
 48. A system as defined in claim 41 inwhich the sampling cell is replaceably removable from the system.